1. Field of the Invention
This invention relates generally to a communication system, and more particularly, to a phased array multiple area nulling antenna utilized in an adaptive nulling communication system for providing independent simultaneous nulling in two or more separate and distinct coverage areas seen by a single nulling antenna.
2. Discussion of the Related Art
Various types of adaptive antenna control systems have been developed to counteract jamming systems or unintentional interference. In general, adaptive antenna control systems reduce the relative strength of a jamming or unintentional interference signal by sensing its direction of origin and greatly reducing the response of the antenna in that direction, while maintaining relatively high gain in the directions of desired emitters. When the desired emitters can be grouped based on signal characteristics within separate and distinct coverage areas or theaters of interest, a common design strategy for maximizing the received strength of the desired signals is to shape the antenna patterns to match the coverage areas. However, in a stressed environment, the need exists to provide independent simultaneous nulling in each of these coverage areas.
In prior art high performance adaptive nulling systems, a nulling process is used to process, at high speed, a relatively small number of signals from a multi-beam antenna. U.S. Pat. No. 5,175,558 describes one such process having an associated multi-beam antenna subsystem which has been implemented for military communication satellites. In the prior art, a physically separate mechanically steerable multi-beam antenna and nulling subsystem is used for each coverage area or theater of interest which has its own distinct signal characteristic. This results in an additional increment of antenna weight, power and cost for each theater to be serviced. In addition, since each theater generally will have a different size or shape, each multi-beam antenna aperture must be custom designed, in advance, for the expected dimensions of each particular theater. This results in higher costs and non-uniformity for each system; it also results in sub-optimum performance if the actual theater shape differs significantly from that anticipated in the design. Still further, by custom designing each antenna, less interchangeability is exhibited, which can result in shorter useful life of the system when failures occur. Thus, the prior art adaptive nulling systems based on multi-beam antennas may not operate as economically, flexibly or reliably as desired.
Active phased array antennas, in contrast to fixed pattern multi-beam antennas, could offer greater flexibility in this application. Phased array antennas can be reconfigured electrically at any time to respond to changes in the shape of an area to be covered, and they are readily expandable to provide simultaneous coverage of multiple areas using a single aperture. Furthermore, in applications like satellite platforms where the total surface area available for mounting antennas is at a premium, sharper resolution and better control of the antenna patterns can be achieved in every theater by "re-using" one large aperture, than by proliferating small apertures in the same available mounting area, and dedicating each of these small apertures to a single theater. However, the need to process signals from a relatively large number of phased array elements, and to change the gain and phase of each of them very rapidly in real time, presents a formidable challenge to the processing element which implements the nulling process. For a given processing element capability, the designer is forced to trade hulling reaction time for the pattern-control benefits of larger numbers of elements. In the prior art, practical limitations on the computational speed and throughput of this processing element have placed high performance nulling processes such as that described in U.S. Pat. No. 5,175,558 out of reach for even moderately-sized phased array antenna systems, in weight and power limited applications such as satellite communications.
What is needed then is a phased array based multiple area hulling antenna architecture for use with an adaptive nulling communication system, which can provide independent simultaneous hulling in two or more separate and distinct theaters with a single antenna, and exhibit a level of performance which is commensurate with that of the prior art based on multi-beam antennas. This will afford all the interference-reducing benefits of the high performance processes, while eliminating the need for multiple antenna apertures to cover multiple theaters, reducing weight, power and cost required for covering additional theaters, allowing real-time adjustment of coverage areas, and providing a high level of redundancy which is advantageous for providing more reliability in the system. It is, therefore, an object of the present invention to provide such a phased array multiple area nulling antenna for use with an adaptive hulling communication system.